The present invention relates generally to systems of the type wherein an information signal and pilot signals are recorded directly on a rotating recording medium. More particularly, the invention relates to a system in which a main information signal is recorded directly along a main information track in a period wherein pilot signals are recorded along pilot signal tracks in a manner such that there is no interference disturbance between the main information signal and the pilot signals.
A new "Information signal recording and reproducing system" disclosed in the specification of U.S. patent application Ser. No. 785,095, now U.S. Pat. No. 4,331,976, of which assignee is the same as that of the present application has been developed and reduced to practice. According to this developed system, in the recording system thereof, pits are formed in accordance with the main information signal such as television video signal, being recorded along a spiral track on a recording medium of flat disc shape, without forming a groove therein, and, in the reproducing system, a reproducing stylus traces over and along this track thereby to reproduce the recorded information signal. In this developed system, pilot or reference signals are recorded on or in the vicinity of a track of the information signal such as video signal on a rotary disc. At the time of reproducing, the information signals are reproduced together with the video signal, and tracking servo control is carried out so that the reproducing stylus traces accurately along the track in response to the reproduced reference signals.
By the use of this developed system, since the recording track has no groove, there is no possibility whatsoever of the reproducing stylus or the recording medium being damaged, and the stylus can trace the same portion of the track repeatedly a plurality of times, whereby a special reproduction such as still, slow motion, or quick motion reproduction becomes possible.
In the above described system, in order not to produce any disturbance relative to the information signal, the pilot signals are recorded during a period corresponding to, for example, the horizontal synchronizing signal duration of a television video signal. Then, in the case where, with the use of the above described system, an information signal and an audio signal are recorded, in order to make possible reproduction with interchangeability with a recording medium on which a television video signal has been recorded as an information signal, the pilot signal is recorded in the time interval (hereinafter referred to as an artificial horizontal synchronizing signal duration) corresponding to the horizontal synchronizing signal of the television video signal. The audio signal to be recorded is rendered into a digital signal by modulation such as pulse code modulation (PCM) or difference modulation and is further digital modulated (encoded) by a digital modulation method such as modified frequency modulation (MFM).
The recording operation has heretofore been carried out by frequency modulating the carrier wave with the information signal. For this reason, in the case where there are two channels for the audio signal, the audio signal can be readily recorded on and reproduced from the recording medium with a broad dynamic range and high quality. However, when the number of channels of the audio signal becomes, for example, four channels, the transmission bit rate becomes twice that in the case of two channels. For this reason, the frequency of the carrier wave in this case must be set at a considerably high value at the time of frequency modulation, and a considerably wide band is required for recording and reproducing. In actual practice, however, there is a limit to the width of the recording/reproducing band owing to the characteristics of the recording medium itself and the organizations of the recording apparatus and the reproducing apparatus.
Furthermore, even in the case of two audio signal channels, if, for example, the recording medium of disc shape is designed with a small diameter, and an audio signal of the same time content as in the case of a large diameter is recorded on this small medium, the signal recording/reproducing band cannot be made very wide.
In such cases, a possible solution appears to be the direct recording of a signal obtained by digital modulation without carrying out frequency modulation at the time of recording. When such a direct recording method is used, the required band for recording and reproducing is not as wide as that required in the case where the frequency-modulation method is used. For this reason the above described problem is solved.
However, when the signal obtained by digital modulation is recorded directly in this manner, an undesired frequency component becomes present in the frequency spectrum of the recorded and reproduced signal down to a low-frequency zone including a frequency which is the same as the pilot signal frequency as will be described hereinafter in detail in conjunction with a drawing. For this reason, this undesired frequency component and the pilot signal mutually interfere, whereby the recording and reproducing characteristics deteriorate, and tracking control of the reproducing stylus in the reproducing system cannot be carried out stably in some cases.
Accordingly, a measure which appears to be a solution for this problem comprises compressing the digital modulated signal in time base so that it will not be present in the artificial horizontal synchronizing signal duration and rendering it into a block in every period corresponding to one horizontal scanning period so as to prevent the above mentioned mutual interference. However, when an information signal is prevented from being present in the artificial horizontal synchronizing signal duration in this manner, it is difficult to cause a clock signal for demodulation in the reproducing system to synchronize with a clock signal for modulation in the recording system. Furthermore, since a part in which a signal does not exist is added to the digital signal having two values, the recording becomes one of a total of three values. In this case, signal transmission cannot be carried out as stably as in the case of recording of only two values. Still another problem is that a wide frequency range is required for the signal transmission.